Create pull_request_template.md

.github/pull_request_template.md

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+Before opening your pull request, please answer the following questions.
+
+#### Is this resolving a feature or a bug?
+
+_NB: We do not accept typo fixes. Generally, we do not accept edits to comments (starting with `//`) or minor grammatical and technical edits more generally, but do accept substanative fixes and improvements to the content of documentation comments (`///`) and README files._
+
+#### Are there existing issue(s) that this PR would close?
+
+#### If this PR is not minimal (it could be split into multiple PRs), please explain why the issues are best resolved together. 
+
+#### Describe your changes.